1. Field of the Invention
The invention pertains to the field of image-forming apparatuses. More particularly, the invention pertains to improved apparatuses for forming, at or closer than infinity, a dispersion-free full color image of an object using a holographic analog of a curved mirror.
2. Description of Related Art
In-line infinity display systems capable of forming images at or closer than infinity of an object or a plurality of optically superimposed objects find important application in aircraft flight trainers or spacecraft simulators to provide the user of the trainer or simulator with a realistic xe2x80x9cwindowxe2x80x9d for viewing a simulated environment. One such system is described in my U.S. Pat. No. Re. 27,356, hereby incorporated herein by reference, reissued May 9, 1972, known and sold under the Trademark Pancake Window(trademark). In that system a single curved, spherical beamsplitting mirror is used as the image-forming element. A primary image is directed at the convex side of the mirror, and a birefringent beam-splitter and polarizer array positioned on the concave side of the mirror reflects the image back to the spherical mirror, which then collimates the image for viewing by an observer. Selective polarization of light prevents the primary image from being seen directly by the observer. The arrangement of that system obviates the use of an oblique space consuming beam-splitting mirror across the axis of the spherical mirror so that the optical elements can be assembled into a compact package, thereby substantially increasing the field of view. Since, with the exception of the curved mirror, all the elements of the apparatus are in the form of flat sheets, the assembled package or xe2x80x9cwindowxe2x80x9d has a relatively thin cross-section. However, the curved mirror by its nature cannot be reduced to flat sheet form and therefore contributes somewhat to the bulk of the apparatus.
Another limitation of the system described in that patent is the fact that the light rays of the primary image must first pass through the optically significant thickness of the curved glass substrate of the spherical beam-splitting mirror before undergoing collimation. The curved glass substrate acts as a meniscus lens, imparting power to the image and introducing spherical and other optical aberrations to the system.
The present invention provides an improved image-forming system of the holographic type, having a large exit pupil, wide angular field-of-view, being compact, light in weight and of inexpensive construction. Additionally, the arrangement of the present invention eliminates the increase in apparatus size caused by the inherent sagitta of the curved mirror and also eliminates the optical aberrations caused by the meniscus lens effect of the curved mirror substrate thickness. Both these defects are overcome by replacing the curved mirror of the prior image-forming apparatus with a reflection-type holographic analog of a curved mirror.
The present invention, actually manufactured and evaluated, permits the multiplexing of a number of in-line infinity display systems surrounding an observer or observers without generating objectionable lunes, (item 50 FIG. 6). Another major advantage of this new full color holographic in-line infinity display system is that the closeness of the now flat holographic spherical mirror analog beamsplitter 12 to the image reflecting flat beamsplitter 14 and all other elements results in extremely wide fields-of-view, whereas the use of a tilted or oblique beamsplitter severely limits the field-of-view. In the course of experimenting with a holographic mirror analog in an in-line holographic infinity display system (FIG. 4), I discovered, unexpectedly, that the dispersed illumination, after being reflected from the plane beam-splitter back to the analog, was effectively filtered by the holographic analog. The analog reflected and collimated the narrow bandwidths of illumination preferential to the hologram bandwidths and the remaining illumination passed partially through the analog towards the source of illumination and partially back through the flat beamsplitter. This portion of the collimated illumination having been polarized in a manner such that it was blocked by the second polarizer. As a result, the observer viewed a collimated dispersion-free version of the primary image. The holographic mirror acted as a reflection filter, selecting and collimating the narrow bands of illumination from the broad band source illumination reflected by the flat beamsplitter 14, and since the holographic analog was in the form of a thin, flat transparent sheet, the thickness and bulk of the apparatus package was reduced, and no optical power or aberrations were introduced because of the meniscus lens-like effects of a curved glass mirror.
In summary, the present invention is an image-forming apparatus comprising a reflection-type holographic analog of a curved mirror 12. A first polarizer 11 is disposed on the side of the analog corresponding to the convex side of the mirror. A first quarter-wave plate 13 is disposed on the side of the analog corresponding to the concave side of the mirror, and a plane beam-splitting mirror 14 is disposed on the side of the first quarter-wave plate remote from the analog. A second quarter-wave plate 15 is disposed on the side of the beamsplitting mirror remote from the first quarter-wave plate, and a polarizer 16 is disposed on the side of the second quarter-wave plate remote from the beam-splitting mirror. The first and second quarter-wave plates have their fast axes oriented with respect to each other at a first integral multiple of 90xc2x0, and the polarizer has its direction of polarization oriented at a second integral multiple of 45xc2x0 with respect to the fast axis of the second quarter-wave plate. In a preferred embodiment of the invention another polarizer is disposed on the side of the analog corresponding to the convex side of the mirror, the direction of polarization of said other polarizer being oriented 45xc2x0 with respect to the fast and slow axes of the first quarter-wave plate.
Standard in-line infinity display systems, due to multiple internal reflections, do generate a xe2x80x9cghostxe2x80x9d image of from 2% to 4% of the brightness of the wanted image. Ghosts although very low in brightness and almost invisible in daylight or bright scenes are undesirable in dusk or night scenes. In this invention such ghosts can be almost entirely eliminated by coating the first endplate 43 closest to the screen 41 not only with an anti-reflection coating on the screen side but also with a xe2x80x9csteppedxe2x80x9d reflection coating of bandwidths that lie between the bandwidths of the analog spherical mirror. In this manner all bandwidths of RGB that are not used in the display system are rejected by the endplate 43 closest to screen 41 so that only the desired bandwidths are operated on by the invention. It is also evident that the same result could be obtained by merely filtering out all RGB bandwidths not desired at the projector.
The present invention provides an improved image-forming system of the holographic type, having a large exit pupil, wide angular field of view, being compact, light in weight and of inexpensive construction. Additionally, the arrangement of the present invention eliminates the increase in apparatus size caused by the inherent sagitta of the curved mirror and also eliminates the optical aberrations caused by the meniscus lens effect of the curved mirror substrate thickness. Both of these defects are overcome by replacing the curved mirror of the prior image-forming apparatus with a reflection-type holographic analog of a curved mirror.
This invention, actually manufactured and evaluated, permits the multiplexing of a number of in-line infinity display systems surrounding an observer or observers without generating objectionable lunes (see FIG. 6). Another major advantage of this new full color holographic in-line infinity display system is that the closeness of the now flat beamsplitter mirror to the image reflecting flat beamsplitter and all other elements results in extremely wide fields of view, whereas the use of a tilted or oblique beamsplitter severely limits the field of view. In the course of experimenting with a holographic mirror analog in an in-line holographic infinity display system, I discovered, unexpectedly, that the dispersed illumination, after being reflected from the plane beam-splitter back to the analog, was effectively filtered by the holographic analog. The analog reflected and collimated the narrow bandwidths of illumination preferential to the hologram bandwidths and the remaining illumination passed partially through the analog towards the source of illumination and partially back through the flat beamsplitter. This portion of the collimated illumination having been polarized in a manner such that it was blocked by the second polarizer. As a result, the observer viewed a collimated dispersion-free version of the primary image. The holographic mirror acted as a reflection filter, selecting and collimating the narrow bands of illumination from the broad band source illumination. And since the holographic analog was in the form of a thin, flat transparent sheet, the thickness and bulk of the apparatus package was reduced, and no optical power or aberrations were introduced because of the meniscus lens-like effects of a curved glass mirror.
In summary, the present invention is an image-forming apparatus comprising a reflection-type holographic analog of a curved mirror. A first polarizer is disposed on the side of the analog corresponding to the convex side of the mirror. A first quarter-wave plate is disposed on the side of the analog corresponding to the concave side of the mirror, and a plane beam-splitting mirror is disposed on the side of the first quarter-wave plate remote from the analog. A second quarter-wave plate is disposed on the side of the beamsplitting mirror remote from the first quarter-wave plate, and a polarizer is disposed on the side of the second quarter-wave plate remote from the beam-splitting mirror. The first and second quarter-wave plates have their fast axes oriented with respect to each other at a first integral multiple of 90xc2x0, and the polarizer has its direction of polarization oriented at a second integral multiple of 45xc2x0 with respect to the fast axis of the second quarter-wave plate. In a preferred embodiment of the invention another polarizer is disposed on the side of the analog corresponding to the convex side of the mirror, the direction of polarization of said other polarizer being oriented 45xc2x0 with respect to the fast and slow axes of the first quarter-wave plate.